Re: c++ class design: where to put debug purpose utility class?

James Kanze <>
Tue, 1 Jul 2008 01:08:29 -0700 (PDT)
On Jul 1, 4:44 am, phlip <> wrote:

Ian Collins wrote:

James Kanze wrote:

Phlip wrote:

That should map onto C++ classes -

Why? That's the first time I've heard that. (There are a lot
of cases where they do map onto C++ classes.)

Um, we agree again!

Unit tests (and many TDD tests) should target "units", which
are code elements that are clearly delimited and accessible to
inspection. Failure of a true Unit Test must implicate nothing
outside its target unit.

In an ideal world. In practice, of course, it's impossible. A
unit test may fail because of an error in the code its testing,
but it may also fail because of an error in the test itself, in
the test framework, in the framework used by the code it's
testing, in the compiler, in the OS, in the machine hardware...
(Hopefully, most of the time, we can safely assume that the
error is either in the code being tested or the test. At least
until proven otherwise. Although when I first started using C++
professionally, over half the errors detected by my unit tests
were due to errors in the compiler.)

Classes should also be clearly delimited and accessible to

You'll have to define your terms better. Classes should
definitely be clearly delimited, and have rigorously defined
interfaces and behavior. So should units which aren't classes.
But what do you mean by "accessible to inspection"? Code
review? ("Inspection" seems to be a human activity here;
although some forms of inspection can be mechanized, surely none
to a sufficient degree to ensure quality in themselves.)

That does not mean units _must_ be classes. For example, if I
code-reviewed your unit test and said, "This is a bad test
because the unit it addresses is not one class," I have
brought nothing to the code review.

Most unit tests do indeed address classes, as a happy
coincidence. A pattern of _all_ the tests not focusing on
classes should raise a warning...

And now we're in agreement. Although I think it somewhat
depends: in a library of mathematical functions, there may not
be many classes, so tests will naturally not focus on classes.
But at least in the domains I'm active in, most "units"
correspond to a single class (and the second largest group,
albeit very much smaller, are units which consist of a very
small number of classes working very closely together).

Here's where we differ. The above assumes you have a design
which defines your classes.

I am so smart that I can heroically define all my classes in
my head before coding them. Etc...

And that I don't believe. I've never met anyone that brilliant.


I am not open to any braggadocio concocted rationales why what
we are doing is somehow vaingloriously wrong, and that all 7
guys are somehow too stupid and /fanfaron/ not to notice it.

The braggadocio is in the part I clipped. That you're all so
brilliant that you can do all the design work in your head,
think of all the test cases in a few days, without any written
support, etc. (Of course, it's a pretty small project, from
what you describe. But even then.)

James Kanze (GABI Software)
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